net_sched: check noop_qdisc before qdisc_hash_add()

Dmitry reported a crash when injecting faults in
attach_one_default_qdisc() and dev->qdisc is still
a noop_disc, the check before qdisc_hash_add() fails
to catch it because it tests NULL. We should test
against noop_qdisc since it is the default qdisc
at this point.

Fixes: 59cc1f61 ("net: sched: convert qdisc linked list to hashtable")
Reported-by: NDmitry Vyukov <dvyukov@google.com>
Cc: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: NCong Wang <xiyou.wangcong@gmail.com>
Acked-by: NJiri Kosina <jkosina@suse.cz>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sched/sch_generic.c

@@ -794,7 +794,7 @@ static void attach_default_qdiscs(struct net_device *dev)
 		}
 	}
 #ifdef CONFIG_NET_SCHED
-	if (dev->qdisc)
+	if (dev->qdisc != &noop_qdisc)
 		qdisc_hash_add(dev->qdisc);
 #endif
 }